On the surfaces of domestic electric devices or interior devices for motor vehicles, a drawing process is applied for various purposes to improve an outward appearance or a tactile impression, shield a glare and prevent a slip. As a grain, patterns of various fine forms are used such as the textures of leather, grain, rock and pebble, pear-skin lacquer, a geometric pattern, etc. In order to form the grain on the surface of the resin product, the grain is given to a metal mold by an etching method or an electro-casting method.
When the grain is applied to the metal mold by the etching method, since the grain is formed by corrosion, the grain can be inexpensively applied even to the metal mold of a large product. However, a fine form is hardly expressed and the same form is repeatedly manufactured.
Further, when the grain in which irregularities are extended in a normal direction of a curved surface of a product is formed by an injection molding, if what is called an under arises that a product taking out direction intersects the curved surface of the product on a plane along a mold opening direction, the irregularities of the grain of a molded product are caught by the irregularities of the metal mold during a taking out operation of the molded product. At this time, a problem arises that when the molded product is taken out from the metal mold by force, the grain of the molded product is broken. Accordingly, in order to avoid the damage of the grain during a mold opening operation, as a drawing gradient (a supplementary angle of 90° of an angle formed by the normal direction of the curved surface of the product and the product taking out direction) along the mold opening direction comes nearer to 0°, the depth of the grain needs to be the shallower in accordance with the change of the drawing gradient so as to avoid the occurrence of the under. Therefore, the depth of the irregularities of the grain needs to be changed in accordance with the curved surface so that in an area of the curved surface of the product where the drawing gradient is nearer to 0°, the depth of the grain is made to be the shallower.
However, when the grain is applied to the metal mold by the etching method, the depth of the irregularities of the grain cannot be continuously changed. Thus, when an etching process is applied stepwise to change the irregularities, seams thereof are exposed. Accordingly, the seams of the metal mold are transferred to the molded product to deteriorate an outward appearance of the molded product.
On the other hand, when the grain is applied to the metal mold by the electro-casting method, a thin resin sheet having a grain embossed is bonded to the surface of a model manufactured in the form of a product, and an obtained model is taken as a master model to form a molding block via an inversion of resin or an electro-casting process. However, when the resin sheet is stuck to the model of a three-dimensional form, since an elongation or distortion arises, or a misalignment arises in the pattern of the seams, a skill of an operator is necessary in order to correct them so as to make them inconspicuous. Further, many processes are required, so that a term of work is lengthened to increase a cost. Thus, only a specialized maker having a private device can use the electro-casting method. Since the form or size of the grain is determined by the grain of the resin sheet to be stuck during the manufacture of the master model, it is difficult to apply an additional pattern to an arbitrary part after the master model is manufactured or to make a change, for instance, enlarge or reduce the grain.
Further, both the methods have a demerit that a chemical treatment is necessary.
Further, as a technique by which a grain of high quality can be more simply formed, JP-A-7-241909 or JP-A-2004-358662 is known. In this technique, initially, a surface measured value obtained by reading a surface form of a leather model is converted into image data in which the depth of the surface form is represented by the density of 256 gradations to digitize the form data of the grain. In accordance with this digital data, processed data is generated by a computer. By using the processed data, the grain is applied to a plane material such as an embossing roll or an embossing plate, or the grain is applied to a three-dimensional object such as a metal mold by a cutting work or a laser beam machining work.
When the processed data is generated from the form data of the grain, if the grain is merely projected on the surface of the three-dimensional object having a three-dimensional form such as the metal mold from one direction to generate the processed data, irregularities of the grain are elongated in an inclined surface of the three-dimensional object to distort the form of the grain. Accordingly, the grain needs to be formed in a normal direction corresponding to a curved surface relative to a surface of a product having the curved surface.
Therefore, as means that forms a grain in a normal direction of the surface of a product, for instance, a modeling software using a voxel data conversion is provided which stacks voxels on a curved surface of the product to generate the grain in accordance with a density corresponding to the depth of the grain in image data.
In this voxel system, form data of the product is converted to voxel data composed of small balls or cubes to determine a pixel of image data of the grain corresponding to each of the voxel data. Then, the density of the pixel is converted to an amount of displacement to stack the voxels corresponding to the amount of displacement on the voxel data. The stacked voxels are converted to polygon data as final data.